All the battles were important and it is hard to say but I would have to say either the Battle of Britain or the Battle of the Bulge. If the Battle of Britain had been won by the Germans, it is still hard to say if a german invasion of England would have succeded, however it would have definatly left England wide open if the valient Royal Airforce had been defeated. The Battle of The Buldge ofcourse just because it was the last great German Offensive. If they had defeated the Americans there It would have atleast slowed the war down and lengthened it for the Germans and maybe given them time to get some of there secret weapons into service in time to make an impact. But if you want to go off that kind of consequense you would have to throw in the Battle of The Atlantic. That was a struggle to keep supplies flowing between American and England and if that had gone sour things could have turned out much differently.